The present invention relates to the field of loudspeaker diaphragms, also known as speaker cones, used for radiating audio frequency sound.
Loudspeaker diaphragms are manufactured in a wide variety of forms by many different processes and utilizing a wide range of materials, depending upon the desired characteristics of the finished loudspeaker and the desired price class. One of the most common materials used for constructing loudspeaker diaphragms is paper. One way of constructing a paper diaphragm is to form an initially flat sheet of paper into a cone and join it together along a more or less radially oriented seam where the edges of the paper meet. The crude cone thus formed is then pressed in a die to conform it to the desired final shape, which includes a central sound radiating cone body and an outwardly extending annular supporting flange. The supporting flange usually is impressed with one or more annular corrugations to increase its flexibility.
The above method of constructing paper cones is generally recognized as being inferior to the more commonly employed method of felting the paper cone from a fiber suspension directly into the desired final form. Properly felted paper cones are free from most stresses inherent in pressed paper cones, and the paper properties can be controlled and varied and thereby modified to obtain the desired acoustical behavior of the finished loudspeaker.
A particular problem inherent to the construction of one-piece felted paper loudspeaker diaphragms is that the required stiffness of the sound radiating portion of the loudspeaker diaphragm is at odds with the required flexibility of the annular supporting flange. As modern loudspeakers have been called upon to efficiently reproduce low audio frequencies at high power, loudspeaker diaphragms have been required to undergo large linear displacements, somewhat like a reciprocating piston. Consequently, it has been necessary to provide the loudspeaker diaphragm with a high compliance supporting flange, that is, a supporting flange having sufficient flexibility to provide a low resistance to displacement, or excursion, of the cone body. At the same time, the high compliance supporting flange must possess sufficient tensile strength to withstand the stresses placed upon it by repeated large excursions without failing prematurely.
Heretofore, it has been recognized that the compliance of the supporting flange of one-piece felted paper loudspeaker diaphragms could be increased by controlling the felting process such that the paper fibers are deposited at a lesser thickness in the region of the supporting flange than in the region of the sound radiating cone body. However, because felted paper is not an especially strong material, it has been found to be nearly impossible to achieve the desired compliance of the supporting flange portion by this method without unduly reducing the tensile strength such that the supporting flange cannot survive intact when operated at low frequencies and high power.
As a remedy, many loudspeaker diaphragm manufacturers have turned to composite construction of the diaphragm from dissimilar materials. For instance, the sound radiating portion, or cone body, is constructed of felted paper in the conventional manner, and the supporting flange is constructed from a second highly compliant and relatively strong material, such as sheet foam rubber (or more likely, a foamed synthetic elastomer). The two separately constructed pieces are assembled and joined together with an adhesive.
While two-piece composite loudspeaker diaphragms such as just described provide high compliance combined with strength, there are still disadvantages associated with that construction. For instance, the foam supporting flange is usually cut from an initially flat piece of sheet foam and pressed in a heated die to form the annular corrugations therein. This results in significant material waste. It is also relatively expensive to assemble a cone and an annular flange while maintaining concentricity of the two pieces. Another disadvantage of die forming foam flanges is that stresses are introduced into the pressed foam, which stresses are usually not evenly distributed in direction or intensity about the annular configuration. Such stressed foam tends to be environmentally unstable, that is, it will tend to warp when the foam is subjected to heating from the environment, thereby deleteriously affecting the acoustic characteristics of the loudspeaker.
The present invention is directed toward overcoming or alleviating the above mentioned problems by providing a loudspeaker diaphragm configuration and method of producing same which allows a felted paper cone body to be provided with a highly compliant yet strong elastomeric supporting flange without requiring separate construction and assembly of the flange and the cone body. The supporting flange is also homogeneous and free of unequal and localized stresses. The result is an essentially one-piece high compliance loudspeaker diaphragm which can operate at low frequencies and high power.